Within the field of electrochemistry, there is a well-known type of an electrolytic cell known as a chlor-alkali cell. Basically this is a cell wherein chlorine gas and sodium hydroxide are produced by passing an electric current through an aqueous sodium chloride solution. The cathodes employed in such chlor-alkali cells are subjected to the corrosive environment of the sodium hydroxide.
Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically impermeable cation exchange membrane, such as the commercially available Nafion.RTM. manufactured by the E. I. duPont de Nemours & Company. Alternatively, the separator can be a porous diaphragm, e.g., asbestos, which can be in the form of vacuum deposited fibers or asbestos paper sheet, as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a noble metal coating to yield what is known in the art as a dimensionally stable anode. One of the unwanted by-products present in a chlor-alkali cell is hydrogen which forms at the cell cathode. This hydrogen production increases the power requirement for the overall electrochemical process, and eliminating its formation is one of the desired results in chlor-alkali cell operation.
Fairly recently, attention has been directed in chlor-alkali cell technology to various forms of oxygen depolarized cathodes. Such cathodes can result in significant savings in the cost of electrical energy employed to operate chlor-alkali cells. Estimates indicate that there is a theoretical savings of about 25 percent of the total electrical energy required to operate chlor-alkali cells provided that the formation of hydrogen at the cathode can be prevented. In other words, about 25 percent of the electrical energy employed in a chlor-alkali cell is used to form hydrogen at the cathode. Hence, the prevention of hydrogen formation by forming hydroxide at the cathode results in significant savings in the cost of electrical power. This is the major benefit of and purpose for oxygen depolarized cathodes.
One known form of oxygen depolarized cathode involves use of an active cathode layer containing porous active carbon particles whose activity in promoting the formation of hydroxide may or may not be catalyzed using precious metal catalyst materials, such as silver or platinum. Unfortunately, however, the pores of such active carbon particles may become flooded by the catholyte liquor thereby significantly reducing their ability to eliminate the formation of hydrogen at the cathode and resulting in decreased operating efficiency. Various attempts have been made to solve this wettability problem, e.g., by providing a backing layer which is hydrophobic to reduce the likelihood of wetting or flooding of the carbon particles in the active layer by the catholyte liquor. Various forms of polytetrafluoroethylene (PTFE) have been utilized for this purpose. Some oxygen depolarized cathodes contain PTFE in both the active layer and in a backing layer to impart hydrophobicity to the desired layer.
In a laminated or sintered electrode having a carbon-containing active layer and a PTFE-containing backing layer, the hydrophobicity of the backing layer resists penetration of alkali from the active layer of the electrode through the backing layer. If alkali bleeds through the backing layer, it interferes with the contact of oxygen (coming from the backing layer side) with the carbon particles in the active layer. If alkali bleeding is very heavy, it covers up the back of the electrode preventing oxygen from contacting the active layer carbon particles.
It would be desirable to have a carbon based electrode which minimizes the problem of alkali penetration of the backing layer and blocking access of the gas to the active portion of the electrode. The present invention provides such an electrode.